In the production of cement raw material mill drying plants are operated in combination with a clinker burning process, in order to supply the exhaust gases formed in a cement rotary kiln, following heat exchangers, to a further heat utilization.
A known milling system can be gathered from a circuit diagram according to FIG. 3. Exhaust gases 3 from a calcining process are supplied by a kiln fan 5 into an exhaust gas pipe 6, via a shutoff device 6a to a roller grinding mill 2. The term kiln fan 5 is used here to define the fan feeding a gas flow to a mill. In combined circuits of cement rotary kilns and heat exchangers with an air-swept roller grinding mill, the kiln or kiln/heat exchanger-fan supplies the kiln gas flow and heat flow to a roller grinding mill, e.g. an air swept roller grinding mill. The term mill fan is used to define the fan, which delivers the gas flow necessary for the mill and which as fan 7 is positioned in the flow direction below the filter 8 functioning as a dust separator. Said fan 7 supplies the exhaust gas 3 with fines or meal 34 produced in the roller grinding mill 2, by means of a dust-exhaust gas pipe 9, which contains a shutoff device 9a, to the filter 8. The fines 34 separated in the filter 8 are delivered by means of not shown conveying systems into silos. The dust-freed exhaust gas 3 is supplied with the aid of the fan 7 following the filter 8 to a not shown chimney flue and ejected.
In a compound operation, in which the milling plant is operated on line with a not shown cement rotary kiln, a shutoff device 15a in a bypass line 15 remains closed. Unusable thermal energy is dissipated in a cooling tower 22, which generally follows the kiln fan 5, or in the roller grinding mill 2, e.g. by water injection.
Individual components of a raw material mixture 33 are supplied from bunkers 31 by means of weighting belts 30 to a feed belt 32 and fed to the roller grinding mill 2. The feed device can be constituted by a flap lock 25, which acts as an air excluder. Coarse material 26 separated in the roller grinding mill 2 is at least partly admixed with the raw material mixture 33 by means of a conveying machinery 24 and is supplied again to the roller grinding mill 2.
In combined operation the milling plant according to FIG. 3 is operated in such a way that the pressure-zero point, i.e. the point of atmospheric pressure, is located shortly upstream of the entry of the exhaust gases into the roller grinding mill 2, so that said grinding roller mill 2, an integrated classifier 13 and the filter 8 operate under a relatively high sub-atmospheric pressure and corresponding sealing mechanisms and stable structures are required, which will be discussed hereinafter.
In direct operation the milling plant according to FIG. 3 is out of action and only the not shown cement rotary kiln is operated. In combined operation the dust produced by the mill 2 and the residual dust from the heat exchanger are separated in the filter 8, so that the term "filter plant" is used. However, in direct operation only the residual dust from the exhaust gas 3 from the not shown heat exchanger is separated. The shutoff device 6a in the exhaust gas pipe 6 and the shutoff device 9a in the dust-exhaust gas pipe 9 are then closed and the exhaust gas flow 3 from the calcining process, following the cooling tower 22, is passed directly via the bypass line 15 into the filter 8, where it is dedusted and fed by means of the following fan 7 and a not shown chimney flue into the atmosphere. In direct operation the fan 7 serves as an exhaust gas fan for the heat exchanger.
For the production of cement raw material independently of the calcining process, e.g. when the cement rotary kiln is out of action or on recommissioning, as no exhaust gas is available, fresh air 4 is supplied by means of a control flap 4a and a hot gas generator 37 to the roller grinding mill 2. The shutoff devices 6a and 15a in the exhaust gas pipe 6 and bypass line 15 are then closed, whereas the shutoff device 9a in the dust exhaust gas pipe 9 between roller grinding mill 2 and filter 8 is opened.
FIG. 4 shows a circuit diagram of a further raw material mill drying system, which is also known as the "three-blower version". For identical features the same reference numerals as in FIG. 3 are used. A first blower, which corresponds to the kiln fan 5 of FIG. 3, but which is not shown, is located in the flow direction upstream of a cooling tower 22 and feeds the exhaust gases 3 from the calcining process to a roller grinding mill 2. A second blower 28, which acts as a mill fan, is located downstream of a multiple-unit cyclone 29 and feeds a partial gas flow, via a return line 16 back into the roller grinding mill 2. The remaining exhaust gas part from the multiple-unit cyclone 29 is supplied by means of a control and shutoff device 14 to a filter 8. The filter 8 is followed by a third blower 38 as exhaust gas fan and which feeds the remaining exhaust gases 3 into a not shown chimney flue as filter exhaust gas. The fines 34, 35 separated in the multiple-unit cylone 29 and in filter 8 are supplied by corresponding conveyer mechanisms 36 to a not shown silo. Reference is made to the remarks concerning FIG. 3 in connection with the devices for the production and supply of a raw material mixture 33 to the roller grinding mill 2.
The mill drying plant according to FIG. 4 can operate independently of the gas and thermal balance of the compound system. By means of a bypass line 15 excess exhaust gas and heat flows from the kiln and calcining process can be bypassed the roller grinding mill 2 with classifier 13 and multiple-unit cyclone 29 and can be dedusted together with the exhaust gases from the roller grinding mill 2 in the following filter plant 8.
FIG. 6 shows an exemplified pressure curve of the combined circuit variant of the plant of FIG. 4. The essential devices with the corresponding association are shown above the pressure curve and given the reference numerals of FIG. 4. FIG. 6 shows that the filter 8 operates in a relatively low sub-atmospheric pressure range and therefore only has to be protected against air infiltrations by relatively limited expenditure. However, the roller grinding mill 2, which is operated with a vacuum of about -50 to -80 mbar, must be virtually "hermetically" sealed to avoid air infiltrations.
Disadvantages also result from the multiple-unit cyclone 29, which is associated with relatively high construction and space consumption costs and is subject to wear, so that there are also high maintenance costs.
A high degree of separation in the multiple-unit cyclone 29 also requires relatively high energy costs and a further disadvantage of the known plant is the splitting up of the end product into coarser fines 35 from the cyclones and finer fines 34 from the filter 8 (FIG. 4).
The flexibility of the milling plant is limited, because the separation level of the multiple-unit cyclone 29 is coupled with the load state (=gas flow) of the mill. The cyclone separation level drops in the case of a partial load, so that there is a rise in the residual dust content in the line 39 following the multiple-unit cyclone 29 to the mill fan 28, which leads to wear phenomena.
Although admittedly the mill drying plant according to FIG. 3 has a simpler pipe layout, lower energy costs and relatively low capital costs due to a compact plant construction. As can be seen from the exemplified pressure curve of the compound circuit variant of the plant according to FIG. 3 shown in FIG. 5, the filter 8 is incorporated into the vacuum range of the roller grinding mill 2, which is approximately -70 to -90 mbar and must therefore be correspondingly designed from the construction and safety standpoints. Filter housings are of considerable size in industrial milling plants. The filter housing must be designed for operational vacuums of almost -100 mbar and for a cold air start to -140 mbar for safety reasons. Considerable constructional and production measures are necessary to achieve the necessary rigidity and prevent a collapse. Even as a result of small cracks in the housing, the following mill fan 7 sucks secondary air into the plant, which acts as a loss on the mill exhaust gas and leads to functional problems.
Pressure fluctuations, which are unavoidable in the enlarged vacuum range, lead to high mechanical loading of the filter housing and to a high susceptibility to cracking and therefore air infiltrations. These directly influence the capacity of the roller grinding mill. In addition, the not "hermetically" sealed points of the roller grinding mill and the classifier, together with corresponding areas in the overall system are potential sources for air infiltrations. The damage points can also not be located from the outside due to a closed insulation.
Another disadvantage is that the exhaust gases 3 must be drawn out of the heat exchanger unit of the calcining process during compound operation through the roller grinding mill 2 and a bypassing of the exhaust gases is impossible (cf. also FIG. 3).
The gas temperature in the dedusting filter 8 cannot be kept independent of the mill exhaust gas temperature. This dependence can alone and together with air infiltrations lead to a local dropping below the dew point and therefore to corrosion in the filter and pipe area. During mill operation no other gases than those from the roller grinding mill 2 can be dedusted.